Some driving assist functionalities that rely upon an electric power assist steering (EPAS) system of the vehicle are designed to have their outputs cause an additional force to be injected by the EPAS system on the steering rack thereof. Examples of such driving assist functionalities include but are not limited to lane assist, torque steer compensation, and driver steer recommendation. Such functionalities can also have a need for their outputs to be limited for worst-case safety needs. For example, an amount of the additional force can be limited dependent upon factors associated with such safety needs.
However, it is known that a limit on the rack output force can make a less than desirable signal to establish safety limits because such limits often have a considerable dependence on driving conditions. For example, in some instances, a rack output force (i.e., outputted assist torque) that is acceptable in curves may not be acceptable in straight driving. A known approach for mitigating this situation of a less than desirable signal is to transform a desired rack force output to a desired driver handwheel torque output. One example of performing such a transformation entails 1.) computing or acquiring a desired rack force injection, 2.) transforming the desired rack force injection to a corresponding handwheel torque injection based upon driving conditions, 3.) summing the corresponding handwheel torque injection with other injection features if there are more than one, and 4.) limiting the resulting value of such summing to worst case safe thresholds (limits in rate, magnitude, etc.). The result of this transformation is handwheel torque offset. This transformation approach inherently adapts to the driving conditions such that the limits on the desired driver handwheel torque output correlate more desirably to worst-case safety limits (e.g., driver-perceived worst case safety limits). Dual torque injection is a known control technique that can be implemented for allowing the driver handwheel torque injection corresponding to the driver handwheel torque offset to be realized as a rack force output that a particular EPAS feature desires.
One shortcoming of the abovementioned approach for mitigating the situation of the rack output force making for a less than desirable EPAS injection signal is that known techniques for transforming the desired rack force injection to the corresponding handwheel torque offset (i.e., step 4 above) are imprecise thereby creating a noticeable difference between the desired rack force injection (i.e., step 1 above) and what will be implemented for rack force as a result of implementing the corresponding dual torque injection for handwheel torque offset. Therefore, a solution that overcomes this shortcoming would be advantageous, desirable and useful.